Method for producing metal powder and metal powder, and electroconductive paste and monolithic ceramic capacitor

ABSTRACT

Metal chloride vapor and reducing gas are brought into contact to form metallic powder, the metallic powder is washed in carbonic acid aqueous solution, and the metallic powder is classified in a liquid phase. In this way, metallic powder, such as nickel powder, in which the content of chloride components is extremely small and the coarse particle content is small, can be efficiently produced.

TECHNICAL FIELD

The present invention relates to a process for production of metallicpowders such as nickel powder which is suitable as a raw material ofconductive paste fillers used in electronic parts such as multilayerceramic capacitors, and relates to the metallic powder, and relates toconductive paste containing the metallic powder and to multilayerceramic capacitors.

BACKGROUND ART

Metallic powder having average particle diameters of 1 μm or less (whichmay be called “ultrafine” particles) such as Ni, Ag, Cu, or Fe are veryuseful as a conductive paste for electrical materials, in particular,for internal electrodes forming material or magnetic material used inmultilayer ceramic capacitors.

Heretofore, noble metallic powders such as Ag, Pd, Pt and Au or basemetallic powders such as Ni, Co, Fe, Mo and W are used in conductivepastes for electrical materials, in particular, as conductive pastesforming internal electrodes for multilayer ceramic capacitors.Generally, a multilayer ceramic capacitor is made by alternatelylaminating ceramic dielectric layers and metallic layers which are usedas internal electrodes, and by forming external electrodes which areconnected to the metallic layer of internal electrodes on both outsidesof the ceramic dielectric layers. Material having a high dielectricconstant material as a main component such as barium titanate, strontiumtitanate and yttrium oxide are used in the dielectric substances. On theother hand, noble metallic powders or base metallic powders mentionedabove are used as metal which forms the internal electrodes. However,since more inexpensive electrical materials are required recently, thelatter base metallic powders are considered to be more useful. Inparticular, a multilayer ceramic capacitor in which electrodes are madeby forming ceramic dielectric layers of ultrafine nickel layers havingparticle diameters of 0.1 to 1.0 μm have been greatly developed.

Ultrafine metallic powders described above can be produced by variousmethods. Recently, vapor phase reduction, in which metal chloride vaporand reducing gas such as hydrogen are contacted, is widely adopted. Bythis method, ultrafine metallic powder having diameters of 1 μm or lesscan be obtained inexpensively, and the diameter of particles can befreely controlled.

However, metal chloride and hydrochloric acid may remain on the surfaceof the ultrafine metallic powder when metal chloride is used as a rawmaterial. These chloride components are difficult to remove by washingwith water. Recently, the size of multilayer ceramic capacitors has beenreduced and the capacitance of the multilayer ceramic capacitors hasincreased, and not only metallic powders having average diameters of 0.4μm but also metallic powders having average diameters of 0.1 to 0.2 μmare required. However, the smaller the particle diameters become, thegreater the chloride components contained in metallic powder produced ina reducing furnace become, and the chloride components are difficult toremove.

These chloride components which are contained in metallic powder reducethe purity of the ultrafine metallic powder as product, promoteoxidation of the metal, and cause the deterioration (rust) of the metal.Furthermore, the chloride components cause deterioration over time ofthe conducting paste, and they may influence the characteristics of theelectrode formed by the paste. Therefore, ultrafine metallic powderwhich has less chloride components and higher purity is required as amaterial for electrodes used in, for example, multilayered ceramiccapacitors.

To remove chloride components contained in metallic power, washingprocess with water can be considered. However, the sedimentation rate ofmetallic powder in water becomes slower as chloride components arereduced by this water-washing, it becomes difficult to separate andcollect the metallic powder by decantation, and as a result, washingefficiency is reduced. Furthermore, yield of the product may be reducedby removing supernatant which contains metallic powder by decantation.Therefore, development of a technique to remove chloride components hasbecome important.

To remove chloride components contained in metallic powder such asnickel powder, a technique in which metallic powder is washed with waterwhich contains organic acid is disclosed in Japanese Unexamined PatentApplication Publication No. 189813/99, and a technique in which metallicpowder is washed with water which contains chelating agent is disclosedin Japanese Unexamined Patent Application Publication No. 346119/94.Although these techniques can remove the chloride componentsufficiently, metallic powders aggregate together to form coarseparticles, or precipitates may adhere. In a forming process for amembrane electrode having a thickness of about 1 to 2 μm, a non-uniformpaste coating may be formed and a multilayered electrode membrane cannotbe formed any longer if these coarse particles and aggregated coarseparticles exist. Therefore, a removing process for the coarse particlesis required using a method such as classification by a liquid cyclone.However, such coarse particles in which metallic powders are aggregatedare difficult to remove. From this viewpoint, the technique describedabove is insufficient.

There is also another problem in that hydroxides of metals are generatedon the surface of the metallic powder by washing the metallic powder inwater. Non-uniform oxide layer is formed on the surface of the metallicpowder if the metallic powder having hydroxide on its surface is dried.As a result, dispersibility is deteriorated and the metallic powderagglomerates together in the case in which conductive paste is formed.Furthermore, in the case in which the conductive paste is used asinternal electrodes in a multilayer ceramic capacitor, cracking ordelamination may occur because the sintering characteristics of themetallic powder are unstable.

Therefore, objects of the present invention are to provide a process forproduction of metallic powder in which chloride components and hydroxideare efficiently removed, chloride components are extremely small, andminimal coarse particles are included as an after-treatment of themetallic powder made by vapor phase reduction which is applied to metalchloride as a raw material, and also to provide the metallic powderproduced by this method, and conductive paste and multilayer ceramiccapacitors formed by the metallic powder.

Objects of the present invention are described more concretely asfollows.

-   (1) To provide a method in which chloride components contained in    metallic powder can be washed and removed.-   (2) To provide a washing method in which the metallic powder can    precipitate immediately even after chloride components are removed.-   (3) To provide a washing method in which chloride components can be    removed without organic compounds remaining on the surface of the    metallic powder.-   (4) To provide a method in which aggregation of the metallic    particles does not occur in the washing process and therefore    classification can be applied efficiently.-   (5) To provide metallic powder having sufficient dispersibility and    sintering characteristics by removing hydroxide and forming uniform    oxide layer on the surface of the metallic powder.

Although the metallic powder produced in the present invention is verysuitable as a raw material of internal electrodes in a multilayerceramic capacitor, it is not limited to this, and this metallic powderis also suitable for other uses such as for sintered materials, magneticmaterials, or catalysts.

DISCLOSURE OF INVENTION

The inventors performed further research to achieve the objectsdescribed above, and it became clear that chloride components andhydroxide on the surface of the metallic powder can be efficientlyremoved by washing the metallic powder obtained by vapor phase reductionin carbonic acid aqueous solution. Furthermore, it also became clearthat coarse particles can be removed extremely efficiently by applyingclassification process in liquid phase after this washing process, andas a result, the metallic powder in which minimal chloride componentsand coarse particles are contained can be produced efficiently, and thusthe present invention was completed.

That is to say, the characteristics of the process for production of themetallic powder of the present invention is to wash metallic powderwhich is obtained by contacting metal chloride vapor and reducing gas,in carbonic acid aqueous solution.

In the process for production of the metallic powder of the presentinvention, it is desirable that chloride components and/or hydroxideremaining on the surface of the metallic powder which is obtained bycontacting metal chloride vapor and reducing gas be removed by washingin carbonic acid aqueous solution. Furthermore, it is desirable that thewashing process in carbonic acid aqueous solution be conducted within arange of pH of 4.0 to 6.5.

In the process for production of the metallic powder of the presentinvention, it is also desirable that metal chloride vapor and reducinggas be contacted to form the metallic powder, the metallic powder be putin pure water to form a water slurry, carbonic acid gas be dissolved inthe water slurry to prepare a carbonic acid aqueous solution, and thewashing process be conducted. Furthermore, in the process for productionof the metallic powder of the present invention, it is also desirablethat the metallic powder obtained by the method described above beclassified in liquid phase.

Furthermore, in the process for production of the metallic powder, it isdesirable that the metallic powder be washed by the method describedabove, dissolved carbonic acid be removed from the aqueous solution, andthe metallic powder be separated and collected.

The metallic powder produced by the process for production of thepresent invention is desirably nickel powder. Furthermore the presentinvention also provides metallic powder obtained by the process forproduction described above, conductive paste which is contains themetallic powder, and multilayer ceramic capacitors including internalelectrodes which are formed of the metallic powder.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a drawing showing a vertical cross section of a productionapparatus for the metallic powder of an Example of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention is explained in detail by way of examples.

The present invention can be applied to metallic powders which areproduced by vapor phase reduction in which metal chloride vapor andreducing gas such as hydrogen, ammonia, or the like are contacted witheach other. Ni, Fe, Co, Cu or the like is desirable as a raw materialfor such metallic powder, and in particular, the present invention isefficient in the production of nickel powder which is used as a rawmaterial for internal electrodes of multilayer ceramic capacitor inwhich remaining chloride components may have a severe effect toefficiency of a capacitor.

A particle diameter of metallic powders of the present invention is notlimited in particular, but it is desirable that the particle diameter be1.0 μm or less, advantageously that it be in a range of 0.05 to 0.5 μm,and more advantageously that it be in a range of 0.1 to 0.4 μm, in arange of 1 to 40 m²/g to describe in specific surface area by BET. Thesemetallic powders can be produced by techniques already known.

First, a process for production of the metallic powder which is appliedby the method of the present invention is simply explained by way ofexample of nickel powder. Nickel powder and hydrogen chloride aregenerated by contacting nickel chloride vapor and hydrogen gas incondition of vapor phase in a reducing furnace. Nickel chloride vaporcan be generated by heating and vaporizing nickel chloride solid.However, from the viewpoint of prevention of oxidation and moistureabsorption and energy efficiency of nickel chloride, it is moreadvantageous that nickel chloride vapor be generated continuously bycontacting nickel metal and chlorine gas, that this nickel chloridevapor be directly supplied to reduction process, and that the nickelchloride vapor be reduced continuously by contacting with hydrogen gasto produce nickel powder.

The particle diameter can be controlled by changing several conditionssuch as flow velocity, residence time, partial pressure, reducingtemperature, and cooling process after reduction of fed nickel chloridegas. Nickel powder which is produced in a reduction process is exhaustedfrom the reduction process with hydrogen chloride, nitrogen gas, andmetal chloride vapor which is not yet reacted. The exhausted mixture isfed into water, oil, or bag filter to be separated and collected. Afterthat, the nickel powder is fed to necessary processes such as washing,classifying, and drying. Alternatively, exhausting and washing can beconducted at the same time.

Chloride components such as hydrogen chloride or metal chloride which isnot reacted yet remain on the surface of the metallic powder which isimmediately after production by the vapor phase reduction mentionedabove. The total amount of these chloride components is, depending onthe conditions of reduction or diameter of the metallic powder, in arange of about 0.005 to 2% by weight (50 to 20000 weight ppm). Thesmaller the particle diameter, the larger the amount of chloridecomponents on the nickel powder. A characteristics of the presentinvention is that the washing process of the metallic powder containingchloride components is conducted in carbonic acid aqueous solution.

In order to remove chloride components, metallic powder was washed withpure water or aqueous ammonia up until now. However, such washingprocess results to remaining hydroxide, such as nickel hydroxide, on thesurface of the metallic powder. If the metallic powder having hydroxideon its surface is dried, non-uniform oxide layer is generated orhydroxide remains, and as a result, dispersibily may deteriorate orproperties may be unstable after preparation into a paste. However, byapplying the washing process using carbonic acid aqueous solution of thepresent invention, not only chloride components but also hydroxide onthe surface of the metallic powder can be removed as described above.

Methods of the washing process are explained concretely next.

-   (1) A method in which after carbonic acid is dissolved into water to    prepare carbonic acid aqueous solution, the metallic powder is put    into the solution and washed.-   (2) A method in which after the metallic powder is put into water,    and carbonic acid is dissolved into the solution to prepare carbonic    acid aqueous solution and the metallic powder is washed.-   (3) A method in which after carbonic acid is dissolved into water to    prepare carbonic acid aqueous solution, the metallic powder is put    into the solution, and the metallic powder is washed while carbonic    acid is dissolved further.

The washing process by carbonic acid aqueous solution can be applied inthe collecting process of produced metallic powder immediately after thereduction process, after the metallic powder is separated and collected,or after the metallic powder is classified. Tap water, well water, orthe like can be used in the washing process of the metallic powder. Inthe case in which high purity water is required, water in which ionshave been removed by ion exchange resin, or filtered water, is usable.Water in which dissolved oxygen is removed is desirable. Ultra-highpurity water having no conductivity exhibits superior washing effects.

In the present invention, carbonic acid is prepared by dissolvingcarbonic acid into the water described above. Since solubility ofcarbonic acid at 25° C., 1 atm is about 0.15%, a carbonic acid aqueoussolution is prepared in a range of 0.05% to saturated concentration, andthe pH of the washing solution is desirably in a range of 4 to 6.5, moredesirably 5 to 6.5.

An Example of the washing process is explained concretely next.

Metallic powder containing chloride components is added to carbonic acidaqueous solution and agitated sufficiently to remove chloridecomponents. After standing, the metallic powder is separated by removingsupernatant by decantation or filtering the solution. If necessary, thisprocess can be repeated. In the washing process by carbonic acid aqueoussolution of the present invention, the metallic powder agglomeratestogether to some extent, and precipitates immediately. Therefore,decantation or filtering can be conducted extremely efficiently, and asa result, chloride components can be removed efficiently.

The washing process by carbonic acid aqueous solution is applied undernormal pressure or pressurized condition. In the case in which thewashing process is applied under pressurized condition, it is conductedby feeding carbonic acid having a pressure of 0.1 to 5 MPa (gagepressure) into water or water and metallic powder suspension in apressure vessel. The concentration of carbonic acid is increased underpressurized condition, and chloride components contained in large amountof metallic powder can be removed efficiently even in small amount ofwater. The temperature of the washing process is desirably in a range of10 to 60° C., and more desirably 20 to 50° C. from the viewpoint ofsolubility of carbonic acid and washing efficiency. Ratio of themetallic powder and the washing water is, depending on the amount ofchloride components, about 50 to 1000 parts by weight of water per 100parts by weight of metallic powder.

The chloride components can be efficiently removed by washing themetallic powder with carbonic acid aqueous solution as described above.For example, in the case in which nickel powder containing 0.05% byweight of chloride components is washed, most of the chloride componentsis removed by washing only once, and after washing is conducted a fewtimes, the chloride components is decreased to 50 ppm or less. Afterwashing process is conducted by the method of present invention, themetallic powder can be separated and collected by precipitating themetallic powder and removing carbonic acid from the carbonic acidaqueous solution containing the metallic powder by heating or reducingpressure.

Furthermore, in the present invention, after the washing processdescribed above, the metallic powder can be classified in a liquid phaseif necessary. In the washing process of known techniques mentioned abovein which water containing chelating agent or organic acid is used, sucha compound is absorbed on the surface of the ultrafine metallicparticles, and the compound is difficult to remove even if washed bypure water. Therefore, the ultrafine metallic particles are agglomeratedtogether to form coarse particles, the agglomerated coarse particles areremoved in a liquid phase classifying process after the washing process,and as a result, the yield of the metallic powder extremelydeteriorated. However, in the washing process of the present invention,although the metallic powder is agglomerated in the washing process, themetallic powder is dispersed again by substituting carbonic acid aqueoussolution with pure water, or by heating and removing carbonic acid afterthe washing process. In this way, from the viewpoint of removing coarseparticles and yield of the metallic powder, efficiency of theclassification can be improved. As a classifying method which can beperformed in a liquid phase, there is a precipitating method, liquidcyclone, or the like.

As described above, the metallic powder is washed by carbonic acidaqueous solution, classified by removing coarse particles if necessary,separated from water by decantation or filtering, and dried to form aproduct. The method of the present invention is superior in efficiencyof chloride components removing, is superior in sedimentation of themetallic powder after being washed, is able to separate and collect themetallic powder more easily, and is able to treat waste liquid moreeasily, compared to the known washing methods by water containingorganic acid or chelating agent. Furthermore, removing processes for theremaining organic compounds on the surface of the metallic powder is notrequired because the compound does not remain after the washing processof the present invention, and even if the compound does remain, it canbe easily removed.

The metallic powder of the present invention produced as described abovecontains little chloride components, specifically, 100 ppm or less,desirably 50 ppm or less, and more desirably 10 ppm or less. The averageparticle diameter is in a range of 0.1 to 0.5 μm, the oxygen content isin a range of 0.1 to 1%, and the metallic powder has a uniform oxidelayer having a thickness of about a few nm on its surface.

Next, in a preparing method for a conductive paste of the presentinvention, an organic dispersing agent is added to the metallic powdersuch as nickel powder of the present invention and is mixed. That is tosay, the conductive paste is prepared by a method in which the metallicpowder of the present invention is added to an organic dispersing agentincluding organic solvent (organic vehicle) such as turpeneol, decylalcohol and cellulose based organic resin such as ethyl cellulose, andis mixed. Furthermore, plasticizer such as phthalate can also be added.In this way, the metallic powder is highly dispersed and does notagglomerate in the conductive paste prepared with the metallic powder ofthe present invention. Therefore, multilayer ceramic capacitors preparedusing the conductive paste exhibits superior characteristics and shortsand delaminations do not occur.

Next, an example of the production of multilayer ceramic capacitors inwhich internal electrodes contain the metallic powder, in particularnickel, of the present invention, is explained as follows.

First, barium titanate as a main component and dielectric ceramiccomposition powder which is containing metal oxide such as magnesiumoxide, dysprosium oxide, barium oxide, calcium oxide, silicon oxide, andvanadium oxide are placed into an organic vehicle which is prepared bydissolving binder such as ethyl cellulose into an organic solvent toprepare a dielectric paste. On the other hand, nickel powder having anaverage diameter of 0.1 to 0.4 μm of the present invention and theorganic vehicle mentioned above are mixed to prepare conductive pastefor an electrode. Next, the dielectric paste is formed into a sheet by adoctor blade method to prepare a dielectric green sheet. On the otherhand, the conductive paste for an internal electrode mentioned above iscoated on the dielectric green sheet by screen printing. After they arealternately laminated, this laminated sheet is cut into fixed dimensionsto form a green chip. The green chip is heated at 250 to 300° C. in theatmosphere to remove binder. Next, it is baked at 1100 to 1300° C. undera reducing atmosphere such as hydrogen to form a ceramic layeredproduct. After that, the ceramic layered product is annealed at 500 to1000° C. under an atmosphere of oxygen to oxidize the dielectric layeragain. Next, external electrodes which are composed of the same metal asinternal electrodes are formed on both end faces of the ceramic layeredproduct so as to connect electrically with the internal electrodes, andplating process is applied on the external electrodes, if necessary, toform multilayer ceramic capacitor.

EXAMPLES

Next, the present invention is explained in detail by way of examples.

Average particle diameter and chlorine content are measured by thefollowing methods.

Measurement of Average Particle Diameter

A picture of the metallic powder (nickel powder) is taken by an electronmicroscope, diameters of 200 metallic particles in the picture aremeasured, and the average thereof is calculated. Diameter of thesmallest circle which encircles a particle is regarded as the diameterof the particle.

Measurement of Chlorine Content in the Metallic Powder

Chlorine content in the metallic powder is measured by combustioncoulometric titration.

A. Production and Washing of Nickel Powder

Example 1

(Production of Nickel Powder by Vapor Phase Reduction)

In a metallic powder production apparatus shown in FIG. 1, nickel powderhaving average diameters of 0.4 μm was produced in the manner explainedbelow. 15 kg of nickel grain (M) was placed in a chlorination furnace 1,the temperature inside the furnace was set to 1100° C., and chlorine gaswas supplied through chlorine gas supplying pipe 14 into thechlorination furnace 1. Nickel metal was chlorinated and NiCl₂ vapor wasgenerated. 10 mol % of the supplied amount of chlorine gas was nitrogengas which was supplied through an inert gas supplying pipe 15 equippedon a lower and side part of the furnace, mixed with NiCl₂ vapor, and themixture gas was fed into reduction furnace 2 through nozzle 17. At thesame time, hydrogen gas was supplied into the reduction furnace 2through reducing gas supplying pipe 21 equipped on top of the reductionfurnace 2 to reduce nickel chloride vapor.

After the reduction process mentioned above, nitrogen gas was suppliedthrough cooling gas supplying pipe 22 equipped on lower and side part ofthe reduction furnace 2 to quench nickel powder (P) and hydrochloricacid which were generated in the reduction, and the nickel powder wasseparated and collected by filtering. The average diameter of the nickelpowder was 0.4 μm.

(Washing of the Nickel Powder)

-   -   The first washing: 1 kg of collected nickel powder was put into        1 liter of carbonic acid aqueous solution in which the pH was        controlled to 4.1 by dissolving carbonic acid, and the solution        was agitated for 10 minutes at 40° C. After 45 minutes of        standing, supernatant was removed by decantation.    -   The second washing: The nickel powder which was separated and        collected by decantation was put into 1 liter of pure water,        carbonic acid was dissolved into the water and the pH of the        solution was controlled to 4.1, and the solution was agitated        for 10 minutes at 40° C. After standing, supernatant was removed        by decantation.    -   The third and fourth washing: The second washing process        described above was repeated 2 more times.

Applying washing process 4 times as described above, to obtain thenickel powder of Example 1.

Example 2

In Example 1, a gas mixture in which dilution ratio of NiCl₂ vapor wasincreased by adding nitrogen gas beforehand, that is, the partialpressure of NiCl₂ was reduced was fed into the reduction furnace 2through the nozzle 17, reduced by hydrogen, and collected by filteringto produce nickel powder having an average diameter of 0.2 μm. Thisnickel powder was washed 4 times in the same way as in Example 1, andthe nickel powder of Example 2 was obtained.

Comparative Example 1

The nickel powder which was produced in Example 1 (Average diameter: 0.4μm) was put into pure water, washing process was conducted 4 times inthe same way as in Example 1, except that carbonic acid was notdissolved.

Comparative Example 2

The nickel powder which was produced in Example 2 (Average diameter: 0.2μm) was put into pure water, washing process was conducted 4 times inthe same way as in Example 2, except that carbonic acid was notdissolved.

Chlorine content of the nickel powder in Examples 1 and 2 andComparative Examples 1 and 2 were measured. The results are shown inTable 1.

TABLE 1 Chlorine content (ppm) Average particle Average particlediameter 0.4 μm diameter 0.2 μm Comparative Comparative Example 1Example 1 Example 2 Example 2 Before washing 880 880 16100 16100 Thefirst washing 60 210 4700 5200 The second washing 17 50 1700 1800 Thethird washing 5 24 60 1500 The fourth washing 3 10 20 620

According to Table 1, the chlorine content was reduced from 880 ppm to60 ppm by the first washing in Example 1. On the other hand, thechlorine content was reduced to 210 ppm in Comparative Example 1. The pHof the solution after the first washing to the fourth washing in Example1 were 5.7 to 5.9. In Example 2, the chlorine content of the nickelpowder was 1.6% by weight before washing process, it was reduced to 60ppm after the third washing, and 20 ppm after the fourth washing withcarbonic acid aqueous solution. On the other hand, in the case of thewashing process with pure water, it was reduced to 1500 ppm after thethird washing and 620 ppm after the fourth washing in ComparativeExample 2.

Comparative Example 3

1 kg of the nickel powder which was produced in Example 1 (Averageparticle diameter: 0.4 μm) was put into 10 liters of 0.02% by weight ofEDTA aqueous solution at 40° C. and agitated for 10 minutes. The nickelpowder was precipitated and supernatant was removed by decantation. Thenickel powder was washed by repeating this process 3 times. By thiswashing process, chloride components of the nickel powder was reducedfrom 880 ppm to 15 ppm.

Comparative Example 4

1 kg of the nickel powder which was produced in Example 1 (Averageparticle diameter: 0.4 μm) was put into 10 liters of 0.02% by weight oftartaric acid aqueous solution at 40° C. and agitated for 10 minutes.The nickel powder was precipitated and supernatant was removed bydecantation. The nickel powder was washed by repeating this process 3times. By this washing process, the chloride component of the nickelpowder was reduced from 880 ppm to 20 ppm.

B. Ability to Separate and Precipitate Nickel Powder in Water Suspension

Next, the abilities to separate and precipitate nickel powder in watersuspension after removing chloride components were observed.

Ability to Precipitate

The suspension of the nickel powder after washing process in Example 1,Comparative Examples 1, 3, and 4 were sampled and nickel powdersuspensions A, B, C, and D (concentration of nickel powder in each was10% by weight) were prepared and precipitation tests were performed. Theresults are shown in Table 2.

Suspension A: pure water+nickel powder (Comparative Example 1)

Suspension B: carbonic acid aqueous solution (pH 5.1)+nickel powder

Example 1

Suspension C: EDTA (0.02%) aqueous solution+nickel powder

Comparative Example 3

Suspension D: tartaric acid (0.02%) aqueous solution+nickel powder

Comparative Example 4

Measurement of precipitation time is explained next.

Each suspension described above was put in a measuring cylinder with aground-in stopper having volume of 1000 ml, the measuring cylinder wasinverted 10 times, and after standing, the positions of the interfacebetween supernatant and precipitate were measured per period.Furthermore, after precipitation was completed, the precipitate layerwas agitated by a glass rod by hand, and the hardness of the precipitatewas observed.

Ability to Separate

In suspension A (Comparative Example 1), B (Example 1), C (ComparativeExample 3), and D (Comparative Example 4) described above, suspensionsB, C and D were washed by decantation with pure water 5 times. Thesesuspensions are substituted by pure water to prepare suspensions again,dispersed by ultrasonic washer. Particle size distributions of nickelpowder in these suspensions were measured by a laser scatteringinstrument (trade name: LS230, produced by Coulter, Inc.). The resultsare also shown in Table 2. D90, D50, and D10 mean accumulated particlesize at 90%, 50% and 10% respectively, in particular, the greater thevalues of D90 and D50 become, the more the metallic powder isagglomerated and dispersibility is lowered.

TABLE 2 Ability to separate and precipitate Ability in classi- ficationand particle size distribution of nickel powder after washingPrecipitation Condition of D90 D50 D10 time (minutes) precipitationSuspension A 1.20 0.55 0.40 No precipitation Smooth after (Comparativeafter 30 minutes standing Example 1) overnight Suspension B 1.35 0.650.30 10 Smooth (Example 1) Suspension C 2.40 1.20 0.70  5 Fixed(difficult (Comparative to disperse Example 3) again) Suspension D 2.451.25 0.70  5 Fixed (difficult (Comparative to disperse Example 4) again)

As is clear from Table 2, the nickel powder washed by carbonic acidaqueous solution of the present invention is slightly agglomeratedtogether, is precipitated immediately, and the precipitated layer issoft and easy to entangle. Agglomeration of the nickel powder isdispelled by washing and substituting with pure water, and the nickelpowder is highly dispersed again. Therefore, wet classification such asdecantation or liquid cyclone is easy to perform. Also, carbonic acidwhich is remaining to the surface of the nickel powder is easily removedby washing with pure water or by heating.

As explained above, in the present invention, metallic powder in whichchloride components and coarse particle are only contained slightly canbe produced efficiently by washing the metallic powder obtained by avapor phase reducing method with carbonic acid aqueous solution.

1. A process for production of metallic powder comprising: formingmetallic powder by contacting metal chloride vapor and reducing gas,washing the metallic powder in carbonic acid aqueous solution.
 2. Theprocess for production of the metallic powder according to claim 1,wherein after metal chloride vapor and reducing gas are contacted to themetallic powder, chloride and hydroxide remaining on the surface of themetallic powder is removed by washing with carbonic acid aqueoussolution.
 3. The process for production of the metallic powder accordingto claim 1, wherein the washing process in carbonic acid aqueoussolution is conducted in a range of from pH 4.0 to 6.5.
 4. The processfor production of the metallic powder according to claim 1, whereinafter the metallic powder which is obtained by contacting metal chloridevapor and reducing gas is fed in pure water to form a water slurry ofthe metallic powder, carbonic acid gas is fed in the water slurry toprepare carbonic acid aqueous solution, and the metallic powder iswashed in the solution.
 5. The process for production of the metallicpowder according to claim 1, wherein the metallic powder obtained by themethod according to claim 1 is further subjected to wet classification.6. The process for production of the metallic powder according to claim1, wherein after the metallic powder is washed by the method accordingclaim 1, dissolved carbonic acid is removed from the washing solution,and the metallic powder is separated and collected.
 7. The process forproduction of the metallic powder according to claim 1, wherein themetal is nickel.